The objective is to elucidate subcellular mechanisms of repair of DNA treated with x-rays and with chemical carcinogens. 1) Endonuclease II of E. coli recognizes damage produced in DNA by x-irradiation. The nature of this damage and the mechanism of repair will be investigated. 2) Endonuclease II makes single strand breaks in DNA treated with methyl nitrosourea and recognizes 0-6 methyl guanine and N-3 methyl adenine, but not N-7 methyl guanine. 3) Endonuclease II makes single strand breaks in DNA treated with acetoxy-acetyl aminofluorene. Details of the mechanisms in 2 and 3 will be investigated. 4) The activity of the enzyme on DNA treated with other carcinogens such as 7-bromo methyl benzanthracine, 3,4 benzpyrene and 4-nitroquinoline N-1 oxide derivatives will be examined. 5) A comparable mammalian enzyme is being purified and its distribution intracellularly and in different normal and tumor tissues will be studied.